JQuery Dialog and Background Freezing

I would like to freeze the background while a jquery dialog box appears. I am using the code below for dialog box.

$("#divGroup").dialog

({

        autoOpen: false,
        modal: false,
        height: 450,
        width: 450,
        resizable开发者_如何学运维: false,
        position: [500, 200],

});

set the modal property to true.

 autoOpen: false,
        modal: true,
        height: 450,
        width: 450,
        resizable: false,
        position: [500, 200],